Was the item humming? Any items that hum prevent sneak.Was the item glowing? Any items that glow prevent hide.Was the item flagged as thief usable? If not, you can't sneak in it.

If you feel an item is improperly flagged, post the exact name and what you feel is wrong with it. (A successful evaluate skill check will help you on that)

Now, with regards to steal...

Never is a strong word. You just need to know what mobs would actually carry something in their inventory. Some carry berries, or cups of water, etc. Great to practice on. You may have to look multiple times to see their inventory, but if they are carrying it, you have a chance to see it.

Get to a high level zone... and a big nasty bad guy is carrying a key. Oh, you could waste a ton of resources trying to take him down... or YOINK! Got it!

Unfortunately I died and lost the gear, so I don't know if it was humming or not. It was a PITA to figure out what piece of gear it was, though, since the error message gives no help. Maybe the error message could be improved to indicate what piece of gear is preventing it (much like the special error message it gives when you are holding a torch/light source). Also, I have no idea how to see if an item was flagged as thief usable.

So if I understand you right, I need meta knowledge to steal in most cases? I need to know that, say, goblins typically can have glimmering potions on them? Or is examine not the right command to use to see what they have? I agree it can be useful if you know what they have on them, but it just seems pointless atm.

You don't need meta knowledge to steal. you need to look at the mob. You can see its inventory. You may have to try multiple times, or it may not have anything. In that case, just try "steal gold NAME".

When thieves "look" at a mob they see what it has equipped, just like every one else does, but unlike everyone else they also can see what the mob is carrying. Now by and large most mobs carry nothing at all. But if they are carrying something, you'll likely see it. As Patches, says, try a few times and if still says you see nothing in their inventory then they probably don't have anything.